In these days, high-precision observation of nano-level or micro-level microstructures has been requested, and in general, a scanning electron microscope (hereinafter referred to as SEM) is used as an apparatus for observing microstructures or the like.
Here, it may be noted that a SEM is a type of electron microscope that can display a three-dimensional image of an observation sample wherein the observation sample is irradiated with an electron beam while the electron beam is scanning the sample; secondary electrons, reflected electrons, characteristic X-rays, fluorescence and the like are generated by the collision of the electron beam with the observation sample; among these, for example, the secondary electrons are detected by a detector, and the brightness corresponding to the electric charge of the detected electrons and the positional information about the electron-irradiated positions are processed to derive the three-dimensional image of the observation sample.
In general, the secondary electron emission gain that represents a ratio between the incident amount of the primary electrons irradiating the observation sample and the emission amount of the secondary electrons emitted from the observation sample by the irradiation of the primary electrons depends on the primary electron acceleration voltage, namely, the incidence energy of the primary electrons. As shown in FIG. 3, the relation between the secondary electron emission gain and the primary electron acceleration voltage is such a function that the secondary electron emission gain has a maximum value in the intermediate region of the primary electron acceleration voltage, and the secondary electron emission gain comes close to zero as the primary electron acceleration voltage comes close to zero or approaches infinity. Here, the primary electron acceleration voltage region where the secondary electron emission gain is 1 or more is designated as the intermediate region B; and of the two primary electron acceleration voltage regions where the secondary electron emission gain is 1 or less, the region where the primary electron acceleration voltage is smaller than that of the intermediate region is designated as the lower region A, and of the two primary electron acceleration voltage regions where the secondary electron emission gain is 1 or less, the region where the primary electron acceleration voltage is larger than that of the intermediate region is designated as the higher region C. Accordingly, in the lower region A and the higher region C of the primary electron acceleration voltage, the secondary electron emission gain is 1 or less and the incident amount of the primary electrons is larger than the emission amount of the secondary electrons, and hence on the sample surface, the amount of electrons having negative charge is relatively increased and the sample surface is negatively charged as the primary electrons are made incident on the sample surface. Additionally, in the intermediate region B of the primary electron acceleration voltage, the secondary electron emission gain is 1 or more, the incident amount of the primary electrons is smaller than the emission amount of the secondary electrons, and hence the amount of the electrons is relatively decreased on the sample surface and the sample surface is positively charged as the primary electrons are made incident on the sample surface.
When the sample to be observed is an electrical conductor and is grounded, the charge built up as described above can be released toward outside the sample. However, when the sample is a body made up of an insulating material or a body surrounded by an insulating material, the charge built up on the surface of the sample cannot be released, and hence an observation of the sample with a SEM is not permitted precisely observing the image of the sample because of such charge-up. In particular, when the primary electron acceleration voltage falls within the intermediate region B, the emission amount of the secondary electrons is larger relative to the incident amount of the primary electrons, and hence the SEM observation image is poor in image shading and the image is displayed to be white as a whole. Accordingly, attempts have been made to prevent the charge-up by decreasing the incident amount of the primary electrons. However, when the incident amount of the primary electrons is decreased, the image resolution is decreased to blur the image. On the other hand, when the primary electron acceleration voltage falls within the lower region A or the higher region C, the sample surface is negatively charged, and hence the negative charge built up on the sample surface deforms the trajectory of the primary electrons incident from the electron gun to inhibit accurate measurement.
Accordingly, when the sample surface to be observed is made up of an insulating material, attempts have been made, for the purpose of preventing the charge-up, to release the charge built up on the sample surface by vapor deposition, on the sample surface, of carbon (C), aluminum (Al), platinum (Pt), or the like.
Additionally, Patent Document 1 has proposed, for the purpose of preventing the charge-up, a scanning electron microscope in which the sample surface is irradiated with the primary electrons at an acceleration voltage at which the secondary electron emission gain of 1 is attained (Patent Document 1). Such an apparatus provides a secondary electron emission gain of 1, namely, the incident amount of the primary electrons is equal to the emission amount of the secondary electrons, and hence no charge is built up on the sample surface to enable prevention of the charge-up.
Further, Patent Document 2 has proposed a technique in which the back side opposite to the sample surface is irradiated with an ion shower, and thus the sample surface negatively charged with electrons is neutralized by the ion shower (Patent Document 2).
On the other hand, when microstructures are observed, observation has also been conducted with a transmission electron microscope (hereinafter referred to as TEM) in addition to a SEM.
Here, it may be noted that a TEM is a type of electron microscope that can display a two-dimensional image of an observation sample wherein the observation sample is irradiated with an electron beam, the electron beam is allowed to transmit through the sample, the transmission amount of the electron beam that varies with the observation position of the observation sample is detected, and the two-dimensional image display of the sample can be conducted by processing the variation of the transmission amount and the observation positions irradiated with the electron beam.
Because the image is observed by irradiating the observation sample with electrons and by allowing the electrons to transmit through the sample, the observation is conducted in such a way that the target sample is cut as thin as possible or the target sample is thinly applied on an electron-transmitting film. Additionally, a sample for a TEM is sliced to a thickness of approximately 100 nm or less so as to allow electrons to transmit through the sample. Further, when a sample is, for example, a biological sample, such a biological sample generally contains a large water content, the water content thereof is evaporated instantly after the sample is placed under vacuum and the shape of the sample is also deformed, and hence complete drying of the sample is needed.
Patent Document 1: JP Patent Publication (Kokai) No. 3-163736 (1991)
Patent Document 2: JP Patent Publication (Kokai) No. 2-15546 (1990)